The present invention relates generally to improved carbons for electrochemical cells, and more specifically, to modified carbons and carbon electrodes, as well as energy producing electrochemical cells, and energy consuming cells for electrochemical synthesis equipped with such carbon electrodes.
Modern technology has placed increased emphasis on producing electrochemical power sources having improved reliability, low weight, small size, high power and longer life. Power sources meeting these criteria readily find applications in portable electronic devices, aircraft, entertainment devices, emergency lighting, wrist watches, hearing aids and calculators, to name but a few.
Various high voltage, high energy-density electrochemical cells have been the subject of recent investigations. Much of the work in this field has involved batteries comprising highly reactive metals, such as lithium. Lithium batteries comprising "soluble cathodes" include the oxyhalide systems lithium/thionyl chloride and lithium/sulfuryl chloride, and the like. Closely related is the lithium/sulfur dioxide system. The lithium/sulfur chloride primary system, for example, employing lithium/sulfuryl chloride primary system, for example, employing LiAlCl.sub.4 as the conductive electrolyte salt and porous carbon black cathodes have been of special interest because of their high open circuit voltage, excellent low temperature performance and greater construction and packaging efficiencies.
During cell discharge, the lithium anode is oxidized and the liquid sulfuryl chloride is reduced at the carbon cathode so that the overall cell reaction is of the type. EQU 2 Li+SO.sub.2 Cl.sub.2 .fwdarw.2 LiCl+SO.sub.2
Despite the important advantages of the lithium oxyhalide systems, there are performance problems which have hindered their wide acceptance. For instance, upon high rate cell discharge, lithium chloride builds up and fills the pores of the carbon cathode; the battery voltage and current outputs decay rapidly at only a low depth of discharge. In addition, after any period on open circuit these cells have an initial low operating voltage (i.e. exhibit a voltage delay) on recommencement of discharge. During storage, particularly at elevated temperatures, the lithium anode tends to develop a passivating film which leads to this delay phenomenon.
Following the empirical method, various approaches have been taken to overcome the foregoing limitations associated with oxyhalide electrochemical cells. Carbon cathode comparison studies for lithium/sulfuryl chloride cells were described by Wade et al in Proc. of the Workshop on the Electrochemistry of Carbons, Vol. 84-85, pages 479-91 (1983) wherein various carbon powders as potential substrate materials were tested. Wade et al concluded that carbon properties, such as surface area, particle size, porosity, ash content and pH should all be important in selecting a carbon black for cathode applications since electrode surfaces may actively participate in electrochemical reaction sequences. Wade et al recognized the presence of naturally occurring reactive groups such as carboxylic acids, lactones, phenols, ketones and quinone moieties. But a general lack of understanding of performance characteristics was acknowledged including the influence of such reactive groups on cell output voltage and cathode capacity etc.
Wade et al also studied Shawinigan acetylene black pretreated with solvents, eg acetone, thionyl chloride and methanol. In addition, starting carbon black powders were acidified or made basic by pretreatments with nitric acid or ammonium hydroxide which treated subtrates were rinsed with water and dried. Cathodes were prepared in the same manner as untreated carbons. According to the authors, these pretreatments were done to partially neutralize surface oxygen functional groups existing on the carbon. Wade et al failed to suggest the addition of nitrogen functionality to carbons in their pretreatments. In addition, acid and base treatments had a lesser effect on final cathode performance than acetone and methanol washing causing Wade et al to conclude that mild pretreatments with acid or base are insufficient to modify surface groups on the carbon substrate.
Walker et al, J. Electrochem. Society, Vol. 132, 1536 (1985), who oxidized Shawinigan carbon black with alkaline KMnO.sub.4 solution, found that cathode operating voltage and specific cathode capacity were severly decreased in a thionyl chloride system.
Most recent improvements have dealt with the anode rather than the cathode. Thus, efforts to offset the passivating coating of the anode provided for use of polymer films to coat the lithium anode. U.S. Pat. No. 4,170,693 discloses the use of methyl and ethyl cyanoacrylates. U.S. Pat. No. 3,993,501 provides for coating the active metal anode with a thin adherent vinyl polymer film. U.S. Pat. No. 4,020,240 discloses the use of a chloroborate electrolyte salt for reducing passivation of the metal anode, and thus help reduce the voltage delay associated with start-up after storage at elevated temperatures. U.S. Pat. No. 4,093,784 discloses improved cell performance by coating the lithium anode with calcium. In contrast, few improvements exist for cathodes, and these have been limited to mostly physical incorporation of catalytic substances such as metal phthalocyanines, as described in U.S. Pat. No. 4,252,875. Likewise, U.S. Pat. No. 4,167,608 discloses high energy-density lithium/thionyl chloride electrochemical cells with particulate copper dispersed in the carbon current collector. This was done to lower the impedance and reduce the risk of explosion associated with elemental sulfur reacting with lithium. Despite the many improvements made in high energy density batteries especially oxyhalide cells, state of the art technology in this field has not entirely lived up to expectations, mainly as a result of safety concerns, specific capacity limitations of carbons, rate capability, self-discharge, limited cell life and storage capabilities, voltage delay, and lack of utility in secondary cell applications.